1. Field of the Invention
The present invention relates to an image exposing apparatus and an image exposing method. More specifically, the invention is directed to an image exposing apparatus and an image exposing method using the apparatus, the apparatus comprises a GaN-based semiconductor laser, and scans over a recording material such as a photosensitive material using an optical beam modulated in response to an image so as to expose the scanned recording material.
2. Description of the Related Art
Conventionally, an image exposing apparatus has been proposed which a optically scans over recording materials with a light source apparatus that employs either a semiconductor laser or an edge emitting type LED (Superluminescent light emitting diode (SLD) etc.) to form images on the scanned upper portions of the recording materials. In the semiconductor laser, GaAs substrate-shaped AlGaInP, AlGaAs, and InGaAsP are employed as structural materials.
In the light source apparatus, which employs the above-described materials, the GaAs which constitutes the substrate is an absorbing material with respect to light emitting wavelengths, and further, a light absorbing material such as InGaAs is employed in a counter electrode. As a result, while the light is independently confined within a light emitting region having a width of, normally, several micrometers, there is a relatively small amount of stray light within regions other than a stripe region due to the effect of the above-described absorbing material.
On the other hand, very recently, in semiconductor lasers, or edge emitting type LEDs, which employ GaN (gallium nitride) material series, and can be gradually used in practical fields, sapphire and SiC are used as substrates. These substrates are transparent with respect to light emitting wavelengths. As a result, stray lights which reaches an edge of a chip, is returned to a region in the vicinity of an activated region by reflections, and this stray light is transmitted through the substrate by a plurality of reflections, so that stray light having various patterns may be produced.
FIG. 9 shows a schematic diagram in the case that such a GaN-based semiconductor laser is employed in a light source of a silver salt type exposing apparatus for performing a spot scanning operation on a silver salt photosensitive material with a polygon mirror and the like.
As shown in FIG. 9, laser light emitted from a GaN-based semiconductor laser 190 is collected by a collective lens 192 to obtain an optical light spot 194 having a predetermined size. However, stray light (so-called “EL light”) 198, the light emitting position and light emitting direction of which are at random, cannot be collected at the spot 194, but may form a blurred pattern 200.
FIG. 10 shows both a relationship between an optical output and a drive current of the spot 194 and a relationship between an optical output and a drive current of the blurred pattern 200. As shown in FIG. 10, in particular, within such a low exposure intensity region of approximately 0.05 mW, which is important in a silver salt exposing system, it can be seen that substantially high power is present in the blurred pattern 200.
In a high-grade silver salt type exposing system having a very high sensitivity as compared with an electro-photographic system using a photosensitive material such as a photosensitive drum, a photosensitive material maybe reacted by this blurred pattern 200, namely the stray light (EL light), causing a fatal defect. For example, in the case that a pattern (for example, stripe pattern and the like shown in FIG. 11A) is formed having a line width substantially equal to the line width of the spot 194, an image is originally formed in a stripe shape, as represented in FIG. 11A. However, image portions among the stripes are colored by the above-explained blurred pattern 200, so that as indicated in FIG. 11B, such an image would be formed, the image quality of which is considerably lowered and the sharpness of which is lowered, and this image of FIG. 11B is different from an expected image.
Since a light amount of a blurred pattern, which is caused by stray light (EL light), is small, problems in area gradation, such as those in an electro-photographic system, seldom arise. However, as to such a photosensitive material having continuous gradation in which a non-sensitive level is very low, such as a silver salt photograph, a very weak background may introduce a blurring of characters and images, which may considerably lower image grades.
Thus, in the silver salt type exposing system especially having the feature of the high-quality image, a reduction of the stray light, which specifically occurs in the GaN-based semiconductor laser, and may have an adverse serious and fatal influence on the images, is required.
Also, as shown in FIG. 12A, in a conventional semiconductor laser other than a GaN-based semiconductor laser, LED-light emission is relatively weak. In the case when the laser light of this conventional semiconductor laser is intensity-binary modulated, such as by pulse width modulation (PWM), if an OFF level is previously set such that a laser drive current becomes no more than a predetermined threshold value “Ith”, a sufficiently high extinction ratio (namely, a ratio of a sensitivity level, at which photosensitive material is sensitive to a predetermined level, to non-sensitive level, at which photosensitive material is not sensitive) can be obtained, and thus, photosensitive materials are not sensitive at the non-sensitive level.
On the other hand, as shown in FIG. 12B, in a GaN-based semiconductor laser, since there is such a region where LED light emission is relatively strong even when a laser drive current is no more than the conventional threshold value Ith, a sufficiently high extinction ratio cannot be obtained. Also, LED-emitted light leaks from a waveguide path, and an entire crystal edge plane emits light. This emitted light may be attenuated while this emitted light passes through an optical system. However, a light amount, which cannot be ignored, may become the above-explained stray light in the vicinity of a predetermined spot of an optical beam and may be present on a photosensitive material. As a result, since the same place is exposed by this stray light plural times, there are some cases in which unexpected color development may occur.
In this case, if the laser drive current at the non-sensitive level is simply set to 0 mA, then the sufficiently high extinction ratio can be obtained and thus the extra color development can be suppressed even for the GaN-based semiconductor laser. However, in this case, a current difference between the sensitive level and the non-sensitive level may increases excessively, such that high-speed modulation cannot be carried out, and a load of a modulation circuit may also be increased.
Further, since photosensitive characteristics differ depending on the type of photosensitive materials, if a sensitive level and a non-sensitive level are set using a specific photosensitive material as a reference, then there are some possibilities that images might not be able to be recorded on these photosensitive materials properly.